This invention relates to currency paper security threads, and more particularly to a capacitive verification device for a security thread embedded within currency paper.
It is known in the art of currency and banknote papers to incorporate a security thread therein. The use of such security threads has increased due to the prevalence of high-resolution, true-color photocopying machines. If modern currency or banknote papers do not have an embedded security thread, the currency can be more easily duplicated with a color photocopier. However, if the security thread is embedded within the paper, it is harder to illicitly reproduce.
The thread may comprise a plastic film having selected aluminum characters formed thereon. The thread is embedded within the currency paper and is not present on either surface of the paper. Such security threads are described in greater detail in U.S. Pat. Nos. 4,652,015 and 4,761,205 to Timothy Crane, both of which are assigned to Crane & Company, Inc.
The security thread described in these patents has printed characters thereon of extreme fine-line clarity and high opacity such that human readability of the printing is possible by means of transmitted light. Yet, the printing remains completely indiscernible under reflected light. To check the authenticity of the thread, the currency is placed under an intense light source and the characters observed by the human eye. However, in commercial situations where such an intense light source is unavailable, thus making a human check for thread presence and authenticity virtually impossible, it is desirable to provide means for automatically determining the thread's presence and authenticity. Various known means for providing verification of the presence and authenticity of the aforementioned security thread are exemplified in U.S. Pat. Nos. 4,980,569 and 5,151,607.
In contrast to the aforementioned security thread having selected metallized characters formed thereon, it is known in the art to use a "solid" security thread. Such thread comprises a polyester substrate having a metal, such as aluminum, vacuum deposited entirely on the polyester substrate to a thickness of, e.g., 100-300 angstroms. Such "solid" threads sometimes have indicia printed thereon indicative of, e.g., currency denomination. However, normally the printing cannot be seen even under an intense light source; therefore, such printing is often times eliminated.
The "solid" thread is normally disposed within the currency in the same manner as the aforementioned selectively metallized threads. That is, the thread extends from top to bottom and transversely across the linear extent of the currency. It is known to use such "solid" security threads within the currency of, e.g., Saudi Arabia. It is also known to sometimes combine the metallization of the "solid" security thread with magnetic material, e.g., iron oxide. The magnetic material may be indicative of currency denomination.
The aforementioned security thread verification devices described and claimed in U.S. Pat. Nos. 4,980,569 ('569) and 5,151,607 ('607) may be used to detect the presence and authenticity of a security thread embedded within currency. The '569 patent discloses a verification device comprising two optical light source/detector pairs disposed on opposite sides of a proffered currency. The source and detector pairs are arranged for transmission and reception of optical energy through the currency if the thread is not present. Also, the source and detector pairs can determine the presence of a counterfeit thread on the currency surface by checking for light reflected off the currency surface. Thus, the '569 patent provides a twofold test wherein the thread, to be genuine, must be detected under transmitted light and not be detected under reflected light. However, the device in the '569 patent may give a false indication of the authenticity of a counterfeit currency when a pencil line is drawn on the currency surface at the normal thread location.
In an attempt to overcome the shortcomings of the '569 patent, the '607 patent discloses a verification device comprising the optical means of the '569 patent in combination with a magnetic detector, magnetic reader, or non-ferrous metal detector. The latter detectors determine the presence of the security thread, while the optical means determines whether the thread is properly within the currency or improperly disposed on either surface. However, heretofore, no known device which effectively verifies the presence and authenticity of the aforementioned "solid" security thread has been provided.
Accordingly, it is a primary object of the present invention to provide an improved security thread detector.
It is a general object to provide a detector that can determine the authenticity of currency at a rapid rate.
It is yet another object to provide a detector that can verify the presence of a security thread embedded within currency paper through use of a capacitive plate and circuitry which senses the change in the amount of charge on the plate caused by a security thread passing in proximity to the plate.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.